Simulated Changes in Atmospheric Transport Climate

Abstract

Atmospheric ?transport climate? characterizes how trace gases are distributed by and within the atmosphere, on average, as a consequence of the interaction of atmospheric flow with tracer sources and sinks. The change in transport climate under global warming is investigated using passive tracers. Experiments with constant localized surfaces sources, pulsed sources, and pulsed boundary conditions are analyzed using a Green-function approach in conjunction with a climatological budget calculation. Under climate warming, interhemispheric exchange times, mixing times, and mean transit times all increase by about 10%. The main transport pathway between the hemispheres via the ?tracer fountain? at the ITCZ is suppressed. Generally less vigorous flow manifests itself in higher tracer burdens in the source hemisphere and in downwind plumes of enhanced mixing ratio close to the sources; these increases are also about 10%. Resolved advection and subgrid transport do not cooperate for all sources in enhancing the near-source mixing ratio. The warmer climate has a reduced cross-tropopause gradient, primarily due to a slightly higher tropopause, which results in a reduction of about 25% in the average tropospheric tracer mixing ratio, and a corresponding enhancement in the stratosphere. A global variance budget shows increased mean and transient tracer variance due to increased generation from strengthened mean gradients near the source and weakened eddy and subgrid transport.